Passively Q-switched dual laser

ABSTRACT

A passively Q-switched laser system uses a common Q-switch cell to control two lasers. The first laser to turn on Q-switches the other laser.

This is a division of application Ser. No. 369,303 filed on Apr. 16,1982.

DESCRIPTION

1. Technical Field

The field of the invention is that of passively Q-switched lasers.

2. Background Art

It is known in the art to accomplish passive Q-switching by means of ableachable material or a saturable absorber such as SF₆. The use of apair of waveguides contained within a single block for forming a pair ofwaveguide lasers is disclosed in U.S. Pat. No. 4,214,319 in whichQ-switching is accomplished by means of a Stark active modulator. Suchmodulators provide switching of the two laser beams at the same time,but have the disadvantages that they are expensive, operate on a limitednumber of CO₂ laser transitions, and have unwanted gas breakdownproblems associated with the required modulating electric fields.

U.S. Pat. No. 3,496,487 discloses a pair of lasers in which one lasercontrols the turn-on time of a second laser by bleaching a dye cellwithin the cavity of the second laser, using the output beam of thefirst laser to do so.

An article entitled Synchronization of Giant Pulse Lasers, by Opower andKaiser in Physics Letters 21, 6, pp. 638-640 discloses a system in whicha first laser triggers two other lasers simultaneously.

DISCLOSURE OF INVENTION

The invention relates to a passively Q-switched pair of lasers employinga single cell for Q-switching, in which the problem of Q-switching bothlasers at the same time is solved by feeding power from each laser beamback into the Q-switch cell so that the first laser to turn on augmentsthe switching process for the other laser.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates in partially pictorial, partially schematic form anembodiment of the invention;

FIG. 2 illustrates an alternative embodiment for coupling the two beams.

BEST MODE FOR CARRYING OUT THE INVENTION

In FIG. 1, two laser waveguides 112 and 114 are contained within acommon ceramic block 110, shown in cross section in this figure. The twolasers in this particular embodiment are RF excited by electrodesperpendicular to the plane of the paper. The electrodes and othercomponents of the laser are omitted from the drawing for clarity. Twolaser beams 122 and 124 are generated within waveguides 112 and 114respectively. These two laser beams are Q-switched within SF₆ cell 128which obstructs the passage of laser radiation from each waveguide,thereby preventing the resonance of that radiation between gratings 142and 144 and end mirrors 162 and 164 which form a pair of opticalcavities. When the intensity of spontaneous radiation in each waveguideresonator is sufficiently strong, the SF₆ absorber is saturated, and theradiation can pass through it and stimulated lasing action can thenevolve. In general, one of the two lasers will reach threshold first.However, for subsequent heterodyne detection of the return signalssubstantial coemission is necessary.

Random jitter is reduced and coemmission of two nearly simultaneouslaser pulses is augmented in this embodiment by feeding back a smallfraction of the laser power within the SF₆ cell so that the first laserto turn on saturates the cell along both optical paths. The two gratings142 and 144 are constructed so that the bulk of the power impinging onthem is returned back in the direction from which it came, but a smallfraction of the power is specularly reflected along beam lines 122' and124' respectively. This effect is normally achieved when the grating isblazed for Littrow operation. The deflected beams strike mirrors 139 and138 respectively, which direct the power back through side window 136 ofcell 128 along a path generally perpendicular to the paths of beams 122and 124. Thus, whichever of the two lasers reaches threshold first, afraction of the power will be tapped off and re directed through cell128 to saturate the absorber in a region intersecting the other beampath.

In this particular drawing, the sample beams are shown as having nomagnification, but those skilled in the art will easily be able toinsert any optics required to expand the sample beams 122' and 124' to asize sufficient to cover any desired portion of cell 128. The use of twomirrors 138 and 139 is also not necessary, and a single plane mirror ora single mirror which is bent may be used depending on the particularembodiment. The frequencies of beams 122 and 124 may be controlled bymeans of piezoelectric actuators 143 and 145 which control the positionof gratings 142 and 144 respectively and therefore control the exactfrequency of oscillation. The frequency of each beam may be monitoredand actuators 143 and 145 can be controlled by conventional frequencymonitoring and control devices known to those skilled in the art.

An alternative embodiment of cell 128 is illustrated in FIG. 2, in whichbeams 122 and 124 enter the cell as before through window 132. Gratings172 and 174, positioned by actuators 143 and 145 as before, direct backmost of the power back along the beam line as in the embodiment ofFIG. 1. A portion of the power is directed along beams 152 and 154, asare beams 122' and 124' in the embodiment of FIG. 1. These beams 152 and154 are deflected by mirror 160 to the other grating and back towardwaveguide block 110. The frequencies of beams 152 and 154 are maintainedat predetermined different values by means of actuators 143 and 145, sothat beam 154 is not parallel to beam 122 and beam 152 is not parallelto beam 124. This lack of parallelism is designed to be sufficientlygreat that neither laser will inadvertently injection control the other.The two beams 152 and 154 overlap the main beams 122 and 124, however,so that bleaching of both channels is achieved by the first laser toturn on and the time jitter between lasers is reduced, as in theembodiment of FIG. 1.

It is known in the art that the pulse width of the output beam of an SF₆Q-switched Co₂ laser tends to be approximately 200 nanoseconds wide. Ifthe laser is used for laser radar ranging applications, then the rangeuncertainly will be limited by the accuracy with which the leading edgeof the pulse can be determined. The range is, of course, determined bymeasuring the time lapse between the exiting pulse and the returningpulse and this time is measured by looking at some predetermined pointon the leading edge of the packet. The pulse shape will be degraded intime so that the returning pulse will have a less sharply definedleading edge than the exiting pulse. The length of a 200 nanosecondpulse packet is approximately 200 feet and only the leading edge or someother representative fiducial character may be used in a conventionallyconfigured radar.

If, however, two different frequencies (f₁ and f₂) are generated in thetwo lasers and these laser frequencies are separated in magnitude bysome convenient amount in the range of tens of megahertz, then the twofrequencies will beat to produce a net heterodyne signal (f₁ -f₂) whichhas a wavelength comparable to or less than the length of the pulsepacket and may be used to enhance the range resolution. If the phase ofthe heterodyne signal formed from returning wave packet is measured,either on a single pulse basis or on a pulse-to-pulse basis (assumingthere is sufficient pulse-to-pulse coherence), then the target locationmay be determined within the length of the packet spread as accuratelyas the phase may be measured.

Incremental phase detection and processing have been employed with cwsystems in the prior art and this technique permits a more precisedetermination of target location. Typically, when the technique isimplemented with cw laser transmitters, either intracavity orextracavity FM modulation is imposed on the laser carrier frequency. TheFM modulation is equivalent to generating the two frequencies f₁ and f₂with the notable exception that the range of the target can only bedetermined with ambiguity related to a multiple of the wavelengthcorresponding to the frequency f₁ -f₂. The limited choice of FMmodulators for CO₂ lasers, their power handling capability and theresulting system ambiguity characteristic renders this system lesseffective in practice than it would appear in principle.

A laser radar system configured using the teachings herein disclosed hasthe advantage that the range ambiguity is removed by the coarse rangeinformation associated with the round trip time-of-flight for theemitted pulse packet of either laser. The relative frequency stabilityof the two pulsed lasers, which is derived from the near totalmechanical, optical and electrtical commonality of the twin laserpackage permits the extraction of additional fine range information bycomparing the alteration in relative phase between the twin laserpackets due to propagation to and reflection from the target ofinterest. This information is extracted from an observation of noteither but both laser pulses at frequencies f₁ and f₂ in an exactlyanalogous manner to the cw-FM modulated system. Without superiorshort-term stability between the two pulsed lasers, and intrapulse phaseinformation could not necessarily be associated with fine targetinformation. A cw CO₂ twin laser constructed according to a recentcopending application has achieved a passive relative frequencystability of 30 KHz for 5 seconds and active stabilization of this laserhas allowed a very highly significant improvement in this baselinefrequency stability. The resulting radar system is simple, compact andrugged by virtue of the fact that the active modulator for the systemhas been replaced by a simple saturable absorbing gas cell whichproduces well behaved reproducible pulsing with a cw rather than pulseddischarge, and which further functions without requiring a high voltageRF modulation source for the modulation crystal.

We claim:
 1. A laser ranging system for measuring round trip time to atarget comprising:laser means for generating simultaneously a pair ofpulsed optical beams at first and second optical frequencies separatedby a predetermined frequency Δf; heterodyne means for detecting aninterference signal at said frequency Δf having an initial phase;heterodyne means for detecting return radiation of said first and secondfrequencies and for generating a return signal at said frequency Δfhaving a return phase; phase means for measuring the phase differencebetween said initial phase and said return phase; means for generating acoarse measurement of said round trip time; and means for combining saidphase difference with said coarse measurement of said round trip time toform an improved round trip time measurement.
 2. A laser ranging systemaccording to claim 1, in which said laser means comprises:a firstoptical cavity having a first optical axis; a second optical cavityhaving a second optical axis; said first and second optical cavitiesconstitute two laser waveguides being contained within a common block; apassive Q-switch cell common to both lasers and disposed to intersectboth said first and second optical axes; means for resonating opticalradiation along said first and second axes; such that the first laser toturn on Q-switches the other laser.